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Chapter 9
Volcanoes
9.1 What causes volcanoes?
• Volcano: an opening in the Earth’s surface
which forms a mountain when layers of
ash & lava build up
• More than 600 active volcanoes
• Most volcanoes are inactive
Two Examples of Volcanoes
• May 18, 1980
– Mount St. Helens in Washington
– One of North America’s most damaging
eruptions
– 59 people were killed—most by landslides &
flooding due to snow on the mountain melting
Two Examples of Volcanoes
• Mount Kilauea in Hawaii
– World’s most active volcano
– It has been erupting continuously since 1983
Volcano Terms
• Magma: melted rock INSIDE THE EARTH
• Melts due to high levels of heat & pressure
• Composed of a mixture of molten rock,
minerals, & gases
• Melting occurs in the mantle
– Which layer of the Earth is this?
Volcano Terms
• Density: m/v (which means?)
• Mass divided by volume
• This means how much something weighs
(m), in relation to how much space it takes
up (v)
• Magma is less dense than the rock which
surrounds it…so…
• Magma rises to the surface of the Earth
How do volcanoes form?
• After long periods of time magma may
reach the surface of the Earth
• Then it will flow out of an opening called a
vent
• Lava will flow out & cool continuously
• This can make large mountains made of
lava & ash packed together
• Crater: steep-walled depression around a
volcano’s vent
Anatomy of a Volcano
• Magma Chamber: area underneath the
Earth where magma comes from
• Lava: magma which reaches the surface
of the Earth
• Vent: opening in the crust of the Earth
where magma is released
• Crater: top of a volcano (bowl-shaped)
which is connected to the magma
chamber by the vent
Where do volcanoes occur?
• Most volcanoes occur along plate
boundaries—areas where plates interact
with one another
• 1. Divergent Boundary: where plates move
apart
• Plates move apart and magma is forced
upward through faults (?)
• Rifts: openings in plates caused by
divergent boundaries
• Ex. Iceland—Mid-Atlantic Ridge
• 2. Convergent Boundary: plates come
together
• Plates move together and the plate which is
more dense will sink and melt under the
other one.
• Then the magma will rise up.
• 80% of volcanoes are convergent
• Ex. Pacific Ring of Fire, Mt. St. Helens
• 3. Hot Spots: areas in the mantle which
are hotter than others
• Plates move over a hot spot and this
causes the plate to melt—then magma
rises to the surface
• Ex: Hawaiian Islands
• Hot spots also form…
• Seamounts: underwater islands
• Once an island is past a hot spot, its
volcanoes will become dormant (meaning
they will no longer erupt)
9.2 Types of Volcanoes
Styles of Eruptions
• Some volcanic eruptions are explosive &
some are quiet.
• There are 3 factors of magma that affect
the type of eruption.
• 1.Trapped Gases
• Gases such as water vapor & CO2 are
trapped in magma by the pressure of the
surrounding magma & rock
• Gas escapes easily from some magma
during quiet eruptions.
• Other eruptions are explosive because the
gas builds up to a very high pressure
• 2. Magma Composition:
• Magma low in silica (an explosive
compound found in magma that traps gas)
is fluid (flows easily) & produces quiet
eruptions
• Basaltic magma
• Lava pours from vents (Hawaii)
• Magma with a lot of silica produces very
explosive eruptions (Mt. St. Helens)
• Silica-rich magma is thick & gas gets
trapped causing pressure to build up
• Granitic magma
• 3. Magma Water Content: High water
content will cause explosive eruptions
• Magma at some converging zones
contains a lot of water vapor
Types of Volcanoes
• The appearance of a volcano depends on
the material that forms it and the type of
eruption.
• 1. Shield Volcano: mountain with gently
sloping sides and a circular base
• Forms when layer upon layer of lava
accumulates after non-explosive eruptions
• Ex: Kilauea (Hawaiian Islands)
• 2. Cinder-Cone Volcano: generally small
volcanoes with steep sides
• These volcanoes throw rock fragments
called tephra high into the air
• The tephra lands around the vent and
begins to pile up creating a steeply-sided,
loosely packed pile.
• 3. Composite Volcano: very large, steepsided mountains
• Begin with extremely explosive eruptions
(magma with high silica content)
• Tephra is thrown into the air
• Next there is a quiet period with lava flows
• This repeating cycle of tephra-lava,
tephra-lava forms the mountain
• Ex: Mt. St. Helens
9.3 Igneous Rock Features
Intrusive Rock Activity
• Because magma rises but only sometimes
reaches the surface, it will intrude on rock
layers which already exist
• These igneous (“born of fire”) structures are
called plutons
• Can range in size from 1km to 100 km
• These areas can be seen in places that have
been eroded or uplifted
• Batholith: largest intrusive igneous rock
bodies
• Form when magma cools underground
before reaching the surface
• One that can be seen is in Yosemite
National Park
• Dikes and Sills:
• Dikes: magma that is squeezed into a
vertical crack which cuts across other rock
layers
• Sills: magma squeezed into a horizontal
crack between rock layers and hardens
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Volcanic neck:
Forms when a volcano stops erupting
Magma hardens inside the vent
Soft cone erodes away first leaving behind
a solid igneous core—volcanic neck
• Ex: Ship Rock, New Mexico
• Many times the tops of volcanoes will sink
into magma chambers.
• The hole left behind is called a Caldera.
• The caldera usually fills with water
• Example: Crater Lake in Oregon
Damage From Volcanoes
• Many snow-capped volcanoes will melt
during eruptions.
• This melting snow causes severe
landslides, mudslides, and flooding.
• Many homes are destroyed due to fire
caused by hot ashes or lava coming into
contact with homes.